1. Field of the Invention
The invention relates to electronic weighing devices. More particularly, the invention relates to an electronic weighing device which is shielded to prevent EMI/RFI emission and to provide EMI/RFI immunity.
2. State of the Art
Electronic weighing devices are widely known in the art and there are many different technologies utilized in these electronic weighing devices. Generally, the electronic weighing technologies utilize a transducer of some type which generates an electrical signal which is proportional to the weight being measured. Most electronic weighing devices use load cell technology. In load cell scales, the applied weight compresses an elastic member which has strain gauges bonded to its surface. The strain gauge is a fine wire or etched foil which undergoes a change in electrical resistance when it is either stretched or compressed. A measurement of this change in resistance yields a measure of the applied weight. Regardless of the technology used, all electronic scales rely on some type of circuit to convert the electrical signal generated by the transducer to a signal which can be used to operated a weight display.
All electronic circuits generate a certain amount of electromagnetic (EM) radiation or radio frequency (RF) radiation and all electronic devices can be adversely affected by this type of radiation. Undesirable radiation of this type is referred to as noise or interference. Thus, the terms EMI and RFI refer to electromagnetic interference and radio frequency interference. EMI and RFI can be classified in three categories: atmospheric noise, galactic noise, and man-made noise. Atmospheric noise is generally the result of thunderstorms and lightening discharges. Sensitive electronic equipment can be affected by a thunderstorm more than one thousand miles away. Galactic noise which reaches earth is primarily the result of solar activity such as sun spots and sun flares. Solar storms can severely affect sensitive electronic equipment. The most preponderant source of EMI and RFI, however, is man-made noise. Every electrical device, including a simple light bulb, is capable of generating a certain amount of noise. Given the ever increasing proliferation of electrical devices throughout the world, man-made EMI and RFI has now been recognized as a serious environmental problem which can adversely affect the operation of all electronic devices.
Society now relies greatly on the accurate function of electronic devices. A malfunction in an electronic device can result in economic loss, can cause a health hazard, or can damage property. For this reason, various industrial standards committees and governmental agencies have adopted specifications for permissible levels of EMI and RFI. These specifications apply to many different electronic devices and provide for EMI and RFI emissions as well as immunity. The emissions specification indicates the maximum permissible amount of interference an electrical device may generate and the immunity specification indicates the amount of ambient interference that an electrical device must tolerate while continuing to function accurately.
There are many situations in which electronic weighing devices should or must comply with EMI and RFI emissions and immunity specifications. Generally, whenever an electronic weighing device is used in the vicinity of other electrical equipment, the emissions and immunity specifications should apply. In some of these situations, interference generated by the electronic weighing device can cause nearby devices to malfunction. For example, in a laboratory where many sensitive electronic devices may be near an electronic weighing device, emissions from the electronic weighing device must be controlled. In other situations, interference generated by nearby devices can cause an electronic weighing device to become unreliable. For example, there are situations where an electronic weighing device is used to constantly monitor the contents of a liquid or gas cylinder. The output of the weighing device is used to determine when the cylinder is near empty, or to determine the rate at which the liquid or gas is dispensed. Ambient RF or EM interference can cause errors in these determinations. Such errors can be costly and/or hazardous. Electronic weighing devices are commonly used in the semiconductor manufacturing industry to monitor gases used in the manufacturing process. An error in monitoring these gases can result in damage to many thousands of dollars worth of semiconductors. Moreover, electronic weighing devices can be used to monitor liquids and gases used in medicine or in environmental control. An error in monitoring these fluids can be hazardous or fatal.
Shielding an electronic weighing device for RFI and EMI is challenging. The weighing devices have moving parts which must not be inhibited and the circuits used in the weighing devices must be accessible to adjust the zero setting and sensitivity of the electronic weighing device.